The proposed studies will examine the cellular and molecular mechanisms regulating isotype-specific polyclonal stimulation of immunoglobulin synthesis and secretion. Selective IgA deficiency (SIgAD), the commonest primary immunodeficiency disease will serve as a clinical model. We have recently demonstrated two distinct mechanisms of B-cell responses t polyclonal activation. One group of B-cell activators has an absolute requirement for macrophages (M) and T-lymphocytes, whereas only T-cells are required for B-cell differentiation by a second group of stimulants. Through kinetic experiments, we shall determine the sequence of cell-to-cell interaction, defining whether the activation signal is passed from M to regulatory T-cells to effector cells or other cell combinations in both M-dependent and M-independent systems. To determine the biochemical basis of M-dependence, we shall utilize a variety of immunopharmacologic agents to substitute for or inhibit M functions. Attention will be focused on specific control mechanisms regulating IgA production including the role of T-cells bearing Fc receptors for IgA (TAlpha cells). We further propose to develop a monoclonal antibody against this unique cell subpopulation and examine its activity when added (with complement) to the in vitro polyclonal B-cell differentiation system, using lymphocytes from normal donors and patients with SIgAD. Cultures of human breast milk lymphocytes have been shown to selectively produce increased levels of IgA when polyclonally activated and moreover secrete a supernatant factor which enhances IgA synthesis and secretion by cultured autologous and allogeneic lymphocytes. We plan to identify and isolate a specific subpopulation of breast milk lymphocytes as well as their soluble products with IgA-specific immunoregulatory activities. These subpopulations and/or soluble factors will be examined in our polyclonally activated B-cell system, using PBL from patients with SIgAD and normal donors in an effort to correct the immunodeficiency in vitro. As we have previously demonstrated the IgA-specific suppressor cells may be found in the peripheral blood of patients with SIgAD, we shall attempt to selectively abrogate this activity in vitro with various agents including ionizing radiation and corticosteroids. Our investigations may ultimately be applied to the development of new treatment modalities for SIgAD which presently eludes specific therapy.